There are at least four ways in which we may think of the day of Pentecost.
First, it was the final act of the saving ministry of Jesus before the parousia. He who was born into our humanity, lived our life, died for our sins, rose from the dead and ascended into heaven, now sent his Spirit to his people to constitute them his body and to work out in them what he had won for them. In this sense the day of Pentecost is unrepeatable. Christmas Day, Good Friday, Easter Day, Ascension Day and Whit Sunday are annual celebrations, but the birth death, resurrection, ascension and Spirit-gift they commemorate happened once and for all.
Secondly, Pentecost brought to the apostles the equipment they needed for their special role. Christ had appointed them to be his primary and authoritative witnesses, and had promised them the reminding and teaching ministry of the Holy Spirit (Jn 14-16). Pentecost was the fulfilment of that promise.
Thirdly, Pentecost was the inauguration of the new era of the Spirit. Although his coming was a unique and unrepeatable historical event, all the people of God can now always and everywhere benefit from his ministry. Although he equipped the apostles to be the primary witnesses, he also equips us to be secondary witnesses. Although the inspiration of the Spirit was given to the apostles alone, the fullness of the Spirit is for us all.
Fourthly, Pentecost has been called – and rightly – the first “revival”, using this word to denote one of those altogether unusual visitations of God, in which a whole community becomes vividly aware of his immediate, overpowering presence.
John Stott, The Message of Acts